(X = 0.5, 0.9) And A ₂ W ₂ (A = Sc, Y) Thin Films And Their Negative Thermal Expansion Properties

The negative thermal expansion (NTE) materials contract with the increasing temperature and these materials become a new branch of materials science in recent years. It has potential applications in electronics, optics, communication, machine, biomedicine, sensor etc. Nowadays, ceramic, powders and filler materials are the hot points of the research. But the materials has a weak point, when the temperature rise up to153℃, it has a phase transition from α-ZrW2O8to β-ZrW2O8. This change, makes the coefficient of thermal expansion changed, which will not only influence the controlling of coefficient of thermal expansion, but also emerge stress in the materials. So, the solution came into being. First, adulterating other elements to control the temperature of phase transition. Second, searching the materials which do not have phase transition in their working temperature. In these two ways, there are some good result of ceramic, powders and filler materials that people have been done, but there is not many information about thin films, and the NTE thin films also have various potential applications in aerospace vehicles, microelectronics, optics and micromachine.The NTE thin films were deposited using different targets by Pulsed Laser Deposition (PLD). The influences of targets, preparing methods and annealing temperature on the film phase, morphology, stress, cohesion between the film and the substrate were studied, and the transmittance of the thin films was measured. And the dielectric constant and transmittance of the thin films were measured. The NTE coefficients of the resulting thin film prepared by different methods were also measured.The microstructure and morphology of the films were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscope (AFM). The stress, elastic ratio and hardness were investigated by laser interference phase-shift technique and nano-indentation. The thermal expansion coefficients of the target were measured by dilatometry.(1) Using coprecipitation function, use ZrO(NO3)2·5H2O, H40N10O41W12·xH2O,N6H24Mo7O24·4H2O as the raw material (AR) to prepare the ZrW2-xMoxO8precursor. Pre-sintering at600℃first, and then sintering at high temperature(x=0.5is1080℃, x=0.9is1050℃).In this way we get the pure cubic α-ZrW2-xMoxO8. Use this material as the target of PLD. Measure the target with High Temperature XRD and Thermo Mechanical Analysis (TMA). It shows that, we get the good NTE material, and the temperature of phase transition has been controlled. As we add Mo into the material, the phase-transition temperature can be control indeed. When W:Mo=1.5:0.5, the phase-transition temperature is decreased to107℃. From room temperature to107℃, the average coefficient of thermal expansion is-23.5×10-6K-1. From107℃to600℃, the average coefficient of thermal expansion is-4.30×10-6K-1.When W:Mo=1.1:0.9, the phase-transition temperature is decreased to90℃. From room temperature to90℃, the average coefficient of thermal expansion is-16.3×10-6K-1.From90℃to600℃, the average coefficient of thermal expansion is-4.86×10-6K-1. when Mo is too much, its heat stability will become worse, it will resolve under300℃.(2) Use self-made ZrW2-xMoxO8(x=0.5,0.9) as the targets to prepare thin films by PLD. We explored the best preparation technology and heat treatment process. The films prepared by PLD films are amorphous, after heat treatment and quenching, we can get α-ZrW2-xMoxO8films. As the abservation of SEM and AFM, when we prepare thin films by PLD, the higher temperature the substrate is, the more flat the films could be.,the higher air pressure is, the thinner the film could be. The amorphous film after heat treatment and quenching, we get the pure cubic α-ZrW2-xMoxO8film. The film is smooth and compact. Calculate lattice parameters from High Temperature XRD and Jade, when W:Mo=1.5:0.5, from room temperature to100℃, the average coefficient of thermal expansion is-1.6864×10-5K-1. from100℃to600℃, the average coefficient of thermal expansion is-7.7848×10-6K-1. When W:Mo=1.1:0.9, From room temperature to100℃, the average coefficient of thermal expansion is-1.6108×10-5K-1. From90℃to600℃, the average coefficient of thermal expansion is-7.8436×10-6K-1.(3) Use Y2O3and WO3as the raw material, after sintering at1000℃, we get the pure Y2W3O12target. The films prepared by PLD films are amorphous, after heat treatment,we can get pure Y2W3O12films. As the abservation of SEM, when we prepare thin films by PLD, the higher temperature the substrate is, the more flat the films could be.,the higher air pressure is, the thinner the film could be. Calculate lattice parameters from High Temperature XRD and Jade, the crystal axis a and b are decrease,and the crystal axis c is increase, the volume of the lattice is increase first, and then decrease. From room temperature to600℃, the crystal axis a average coefficient of thermal expansion is-3.966×10-5K-1, the crystal axis b average coefficient of thermal expansion is4.760×10-5K-1, the crystal axis b average coefficient of thermal expansion is-3.895×10-5K-1,the crystal volume average coefficient of thermal expansion is-3.227×10-6K-1.(4) Use Sc2O3and WO3as the raw material, after sintering at1000℃, we get the pure Sc2W3O12target. The films prepared by PLD films are amorphous, after heat treatment,we can get pure SC2W3O12films. As the abservation of SEM, when we prepare thin films by PLD, the higher temperature the substrate is, the more flat the films could be./the higher air pressure is, the thinner the film could be. Calculate lattice parameters from High Temperature XRD and Jade, the crystal axis a, b and c are all decrease. From room temperature to600℃, the crystal axis a average coefficient of thermal expansion is-9.016×10-6K-1, the crystal axis b average coefficient of thermal expansion is-8.949×10-6K-1, the crystal axis c average coefficient of thermal expansion is-9.538×10-6K-1, the crystal volume average coefficient of thermal expansion is V. It is better than Y2W3O12.